The present invention relates to a contact sheet used with a socket employed for connecting an integrated circuit having spherical terminals arranged in a grid pattern and a board, and more particurlarly to a contact sheet used with a thin socket employed for high-frequency testing or mounting.
In recent years, because of the demands for reduced sizes and higher speed in information processing equipment, the pitches in integrated circuits are increasingly becoming smaller, and the mounting method is shifting from through-hole to surface mounting, and the arrays of terminals are shifting from peripheral arrays to grid arrays. As a result, ball grid array (BGA) devices are becoming the mainstream of packages as they permit surface mounting even for grid arrays.
A socket is used between a BGA type integrated circuit device and a board when conducting a burn-in test or a high-frequency test of the integrated circuit, and the socket is also mounted on a board when replacing an integrated circuit. In a socket, all contacts must be in secure contact with both the terminals of an integrated circuit and terminals of a board in order to ensure sufficient electrical conduction.
On the other hand, in recent years, with the increasing speed of MPUs or memories, the demand for lower inductance with respect to faster clocks has been arising. To meet the demand, it is necessary to minimize the distance over which currents pass in a contact placed between a terminal of an integrated circuit and a terminal of a board.
As a thin connector or socket with a small pitch, a component called "an anisotropic conductive sheet" has conventionally been used. There is known, for example, a conductive sheet in which conductive elastomers or metal wires are arranged in an insulative elastomer (U.S. Pat. No. 3,862,790 and U.S. Pat. No. 4,295,700) and also one in which conductive particles are added (Japanese Unexamined Patent Publication No. 6-82521). FIG. 15 shows a conventional example wherein thin metal wires 31 are embedded in a silicone rubber 30 at a high density.
Furthermore, as conductive sheets to which considerations have been given to the aspects of BGAs that their spherical terminals are soft and their surfaces are made of solder coated with oxide films, there is one with dendrites grown on contact portions (U.S. Pat. No. 5,691,041), one in which cantilevers having protrusions are fixed on a flexible board (U.S. Pat. No. 5,629,837), one in which the surfaces of terminals on a flexible board have been provided with asperities by chemical processing or the like (Japanese Unexamined Patent Publication No. 10-32070), and one in which innumerable protrusions are formed by thermal spraying or the like (Japanese Unexamined Patent Publication No. 10-144440), etc.
Furthermore, as ones with a structure for avoiding contact with ball tips while destroying oxide films on solder surfaces, there is one having Y-shaped contacts (Japanese Unexamined Patent Publication No. 9-21847), and one inserted in finger springs (U.S. Pat. No. 5,702,255 and U.S. Pat. No. 5,730,606).
As the number of terminals of an integrated circuit increases, the force required for insertion increases. There is a contact sheet having a structure in which the foregoing force is reduced to zero, and spherical terminals are pushed laterally later by using a lever or the like (U.S. Pat. No. 5,578,870 and U.S. Pat. No. 5,637,008). As this type, one employing vertically long cantilevers is the mainstream. Moreover, there is a socket described in Japanese Unexamined Patent Publication No. 9-55273 that permits replacement of contact portions while solving the problem peculiar to solder balls mentioned above.
However, in an anisotropic conductive sheet employing the elastomer as shown in FIG. 15, mostly, terminals having flat surfaces provided with gold plating are connected. If this conductive sheet is used for connection with a BGA having spherical terminals made of solder, solder surfaces will inevitably be dented, posing the problem of unstable electrical contact.
Furthermore, in those with asperities or protrusions formed on the surfaces of the terminals on the flexible boards, variations in height of terminals will cause insufficient electrical conduction in some circuits. In the Y-shaped contacts, the distance over which currents pass is relatively long, making them unsuited as contacts for high-frequency testing or mounting even though reliable electrical conduction can be ensured.
The present invention has been made with a view toward solving above the problems, and an object thereof is to provide a contact sheet that enables reliable electrical contact to be obtained by removal of an oxide film from a solder surface, the present invention is capable of accommodating variations in height or size of spherical terminals, prevents collapse of a tip, features a short distance over which currents pass, permits a zero insertion force (ZIF) structure to be implemented as necessary, and also enables a structure for permitting replacement to be implemented.